General copper alloys are stipulated in Bronze Ingots for Castings (JIS H 2203), Phosphor Bronze Ingots for Castings (JIS H 2204), and Brass Ingots for Castings (JIS H 2205). Wrought phosphor bronze (JIS C 5191) and wrought brass (JIS C 2801) are also stipulated. The same components as these alloys are sintered, and the resultant alloys are used as sliding materials.
Generally speaking, sliding materials are used to control coefficient of friction between two opposing bodies. The sliding materials are roughly classified into one group aiming to lower coefficient of friction, such as material for application as an engine metal, and another group aiming to enhance coefficient of friction, such as material for application as a brake.
The sliding material according to the present invention has an objective to lower coefficient of friction and is used specifically for such parts as bearings and washers for vehicles, such as automobiles, and two-wheeled vehicles, and for construction machinery. The sliding material according to the present invention is also used for such parts of general machines.
When the same components as the bronze or phosphor bronze (JIS H 2204, PbCln2 and 3) are sintered to provide a sintered alloy, since these components fundamentally attain the objective as described above, this alloy is therefore used to produce a bearing for an automatic transmission; specifically, an end bearing. The end bearing is used in a one-way clutch of a shift gear in an automatic transmission. The one-way clutch is a sliding member positioned between an outer wheel and an inner wheel of the shift gear of the transmission. When the outer peripheral surface and the inner peripheral surface of the end bearing slide on the outer wheel and inner wheel, respectively, driving force is smoothly transmitted via the sliding member. When either the outer or inner wheel rotates, the load generated in a radial direction of each wheel is supported by the sliding member referred to as an end bearing. The copper-based sintered material is also used to produce a washer of a planetary pinion, which washer is a main structural element of the automatic transmission and is positioned between a pinion gear and a carrier.
Hard particles, which have been added to enhance the wear resistance of bronze and the like mentioned above, are of the following kind: Fe2P, Fe3P, FeB, Fe3B, Co, Co-based self-fluxing alloy, Ni-based self-fluxing alloy, Fe—Cr, Fe—Mn, Fe—Ni, Fe—Si, Fe—W, Fe—Mo, Fe—V, Fe—Ti, Fe—Nb, CuP (Patent Document 1, Japanese Patent No. 3298636); aluminum nitride (Patent Document 2, Japanese Patent No. 3370785); Fe—Mn—Si based hard matter (Patent Document 3, Japanese Patent No. 3929288); Cu—Al intermetallic compound (Patent Document 4, Japanese Unexamined Patent Publication (kokai) No. 2002-256731), and the like.
Since Pb contained in the conventional copper-based sintered sliding material is environmentally problematic, Bi is used as a substitute for Pb, as described for example in Patent Document 5, WO 2005/068671. The material proposed in that document is as follows. The material contains Bi in an amount of 1 to 30%, and hard particles such as Fe2P, Fe3P, FeB, Fe2B and Fe3B and the like in an amount of 10 to 50%. The Bi phase, which is finer than the hard particles, is dispersed in the copper matrix. As an alternative of the fine dispersion, with regard to the Bi phase in contact with the hard particles, the document proposes that the contact length ratio of the hard particles to the length of the entire periphery of the Bi phase be 50% or less.
The Bi-containing Pb-free copper-alloy sintered material has improved seizure resistance because of the following reasons. Bi in the form of a soft phase can be finely and uniformly dispersed in the copper alloy. Adhesion of Bi on iron does not occur. And, Bi has low melting point. This material is at present most advisable for maintaining the seizure resistance of a Pb-free copper-alloy sliding material to an equivalent level of the lead-containing copper-alloy sintered material. The Bi-added sliding material is used for sliding parts of an automatic transmission (Non-patent Document 1: Tribologist (in Japanese) Vol. 53/No. 9/2008, pages 599-604).
It is known to disperse graphite or MoS2, which is a general solid lubricant, in copper-alloy sintered material. Disadvantageously, graphite does not exhibit good sintering property to copper, and MoS2 decomposes at sintering temperature. Therefore, several proposals have been made to solve these problems: In Patent Document 6, Japanese Unexamined Patent Publication (kokai) No. 2006-037178, oxidized Mo and sulfurized Cu are added. In Patent Document 7, Japanese Unexamined Patent Publication (kokai) No. 2005-179692, a sulfate compound and graphite are added. In Patent Document 8, Japanese Unexamined Patent Publication No. 2006-37179, plated MoS2 particles are used. It is also noted that MoS2 and graphite are expensive.